An optical coupler, which couples light emitted from a plurality of light sources, is often employed in a laser device such as a fiber laser. Such an optical coupler includes (i) a plurality of input optical fibers, (ii) a taper fiber having an entrance end surface to which exit end surfaces of the respective plurality of input optical fibers are bonded, and (iii) an output optical fiber having an entrance end surface to which an exit end surface of the taper fiber is bonded. The taper fiber is an optical fiber (i) which has a reduced diameter part in which a core diameter of the taper fiber gradually decreases as a distance is farther from the entrance end surface and (ii) in which the core diameter in the exit end surface is smaller than in the entrance end surface.
In such an optical coupler, it is of importance to reduce an NA (numerical aperture) of light which has entered the taper fiber. This is because the light, which has entered the taper fiber, increases in NA while propagating in the reduced diameter part due to reflection of the light at a boundary between a core and a cladding.
As such, even in a case where an input fiber has an NA substantially equal to that of the output optical fiber, an NA of light which exits the taper fiber, i.e., an NA of light which enters the output optical fiber, can exceed the NA of the output optical fiber. This causes light to be leaked in the vicinity of the entrance end surface of the output optical fiber, and ultimately causes a deterioration in and/or burnout of a jacket of the output optical fiber.
An optical coupler (referred to, in Patent Literature 1, as an “optical fiber combiner”) disclosed in Patent Literature 1 is known as a technique to address such a problem. According to Patent Literature 1, the optical coupler makes good in a reduction in NA of light which has entered a taper fiber (referred to, in Patent Literature 1, as a “bridge fiber”), by providing a graded index (GI) fiber referred to, in Patent Literature 1, as “GRIN lenses”) between respective input optical fibers and the taper fiber.
Note that, in the present specification, an NA of light which propagates in an optical fiber (which includes a taper fiber) refers to a physical value defined by n×sin θ, where θ is an angle between a propagation direction of the light and a central axis of the optical fiber and n is a refractive index of a core of the optical fiber. If an NA of light which has propagated in the taper fiber does not exceed that of an output optical fiber before the light exits the taper fiber, then the light will be confined in the output optical fiber. In contrast, if an NA of light which has propagated in the taper fiber exceeds that of the output optical fiber before the light exits the taper fiber, then the light will leak out of the output optical fiber.